The art of measuring fluid flow, either liquid or gas, is now relatively sophisticated and employs comparatively complex instruments. In solving aerodynamic problems wherein wind tunnels are utilized, it is frequently important to determine the speed of fluid flowing at specific selected points in the flow channel. Flows immediately adjacent to a vehicle model under test may be measured by utilizing pressure pickups mounted flush in the model surface at the desired location and comparing the sensed pressure with the known dynamic pressure of the free stream flow. For other preselected points, devices which have previously been utilized include pitot tubes, vanes, hot wire anemometers, heat flux transducers, and the like. These devices have serious disadvantages in that they must be physically located in the flow at the point where measurement is desired, and consequently distort the local flow and change the true velocity. Secondly, the devices are subject to adverse environmental factors such as excessive pressures and temperatures and may suffer damages therefrom.
With the development of lasers, providing sources of highly coherent light, velocity measurement in a flow field has become possible without affecting or disturbing the flow field. The main principle which underlies the operation of a laser velocimeter is that the wave length of light scattered from a moving object is modified by the motion of that object and the frequency of the light is shifted in a manner equivalent to the Doppler effect. Particles in the flowing fluid will scatter the light beam as they move across the focal point of a lens illuminated by the light beam. The particles must be such that they are suspended in the field and move at the same velocity as and in the same direction as the fluid flow.
A typical laser velocimeter based on this principle focuses an unscattered coherent reference light beam and the scattered light on a photomultiplier tube, and the non-linear action of the photomultiplier tube causes a heterodyning action which produces an electrical output whose frequency is the difference between the frequencies of the two light sources. This frequency shift is directly proportional to the velocity of the suspended particles, and its detection is suitably processed electronically to produce an electrical signal which is converted into a velocity display, such as a periodic wave on an oscilloscope, a side band on a spectrum analyzer, or converted into an electrical signal directly proportional to the velocity. This type of laser velocimeter is generally referred to as a "reference beam" velocimeter.
An improvement in this technique permits measurements to be made in three dimensions by providing three independent receiving systems that are all focused from different directions on the same scattering volume illuminated by the coherent beams. Each of the Doppler-shifted, or scattered beams is coherently mixed with a portion of the reference beam which has not been Doppler-shifted to obtain the velocity in three dimensions, which thereby permits determination of the velocity vector. It is to be noted that in these reference beam systems, the Doppler shift measured is a function of the direction of light collection.
Another known method of fluid velocity detection, generally referred to as utilizing a "crossed beam" (differential mode) velocimeter, employs two unscattered coherent laser beams which are caused to intersect at a predetermined point of interest in the flow field, whereupon the two beams produce a fringe pattern at this point of intersection. A particle passing through this fringe pattern will produce a periodic variation in the amount of light scattered as it passes through the light and dark crests of the fringe pattern. Particles passing through the intersection of crossed laser beams produce a Doppler difference frequency which is independent of the viewing direction.
From the above it is apparent that laser velocimeters provide a means of measuring fluid velocity without the necessity of inserting probes into the flow field. Such velocimeters are limited however to reading the local velocity in a finite area, i.e., the focal point of the reference beam velocimeter optics, or the point of intersection of the crossed beam velocimeter. To move this point requires precision adjustments of the laser beams and/or the receiving optics, which are time-consuming and which requires much testing. The magnitude of this problem will be appreciated if one understands that the unfocused laser beams are approximately 1 to 2 millimeters in diameter, The precision alignment of these beams so that they illuminate the desired finite area or cross at the desired point is quite difficult, and once alignment is obtained, the alignment may be lost by any change in the refraction index of the flow field medium, such as changes in density of the flow medium, such as those caused by the development of shock waves in the medium.
Another condition which complicates beam alignment is encountered whenever it is desired to move the beam to a new area of interest (the focal point of a reference beam velocimeter or the intersection point of a crossed beam velocimeter), since any change in the angle of incidence of the beams with the tunnel window or with the flow medium will change the beam refraction.
Further background will be given by examining the following U.S. prior patents, which appear to represent the closest prior art relating to the present invention, found in the course of a preliminary search:
U.s. pat. No. 3,532,427--Paine PA1 U.s. pat. No. 3,623,361--Funk PA1 U.s. pat. No. 3,809,480--Somerville PA1 U.s. pat. No. 3,825,346--Rizzo PA1 U.s. pat. No. 3,915,572--Orloff PA1 U.s. pat. No. 3,856,403--Maughmer PA1 U.s. pat. No. 3,966,324--Iten